How to create an Inner Glow node effect
Use the Erode Node

A simple Dilate/Erode node in Threshold mode with "Distance" set to the same value as "Edge" does exactly the right thing.


With the math node "Power" afterwards the result matches the image from the tutorial perfectly.



Additional explanation

Linearity of the gradient

The Threshold mode of the Dilate/Erode node has a linear falloff. Here is a proof:
In the viewer-result you see the texture above and the output of the erode node below.

Congruence of the results

Here is a proof that the result of photoshop's inner glow effect is the same as the result of my proposed node graph above. The differences still visible are caused by different image formats and scaling blurriness.